1. Field of the Invention
The present invention relates to a light-emitting element driving device large in internal resistance (series resistance) and particularly to a light-emitting element driving device preferably used for driving a light-emitting element such as a laser element used in laser xerography.
2. Description of the Related Art
In a field of laser xerography using a laser element as a light source, the demand for higher resolution and higher speed has been intensified. The on/off control speed (hereinafter referred to as modulating speed) for controlling the laser element to be driven according to input image data is limited. When one laser light beam is used, the modulating speed cannot but be sacrificed if resolution in a sub scanning direction as well as resolution in a main scanning direction needs to be improved. Therefore, the number of laser light beams cannot but be increased if resolution in a sub scanning direction needs to be improved while the modulating speed is not changed. When, for example, four laser light beams are used, resolution both in the main scanning direction and in the sub scanning direction can be improved to twice on the assumption that the modulating speed is the same as that in the case where one laser light beam is used.
Incidentally, semiconductor lasers are roughly classified into edge emission type laser elements (hereinafter referred to as edge emission lasers) and surface emission type laser elements (hereinafter referred to as surface emission lasers). The edge emission laser has a structure in which laser light is taken out in a direction parallel to an active layer. The surface emission laser has a structure in which laser light is taken out in a direction perpendicular to an active layer. Heretofore, in laser xerography, the edge emission laser has been generally used as a laser light source.
From the point of view of increasing the number of laser light beams, the surface emission laser is however structurally preferred to the edge emission laser for the purpose of increasing the number of laser light beams because the edge emission laser is regarded as having a technical difficulty. For this reason, in the recent years, there has been advanced the development of a device using a surface emission laser capable of emitting a large number of laser light beams as a laser light source to satisfy the demand for higher resolution and higher speed in the field of laser xerography.
Semiconductor laser drive systems are roughly classified into voltage drive systems and current drive systems. Assuming now that a light-emitting element large in internal resistance such as a GaN (gallium nitride) blue laser or a single mode surface emission laser needs to be driven, then a time constant τ(=R·C) decided on the basis of internal resistance R and parasitic capacitance C of wiring increases if the light-emitting element is driven by the current drive system. As a result, the leading and trailing edges of a drive current waveform become very slow so that the modulating speed is reduced. Accordingly, if the modulating speed cannot be improved greatly, there is no merit in use of the surface emission laser as a laser light source particularly in laser xerography.
From this point of view, the voltage drive system is more advantageously used as a system for driving a light-emitting element large in internal resistance than the current drive system. Heretofore, in a driving device using the voltage drive system, the light-emitting element has been driven by a voltage by an element lower in output impedance than the light-emitting element (e.g., see JP-A-2001-036186) in order to increase the modulating speed. In the voltage drive type laser driving device, feedback control is performed so that a drive voltage Von corresponding to the set intensity of light is applied to the semiconductor laser when the semiconductor laser is switched on, and a bias voltage Vbias not higher than an emission threshold is applied to the semiconductor laser when the semiconductor laser is switched off.
On the other hand, in a device for driving laser elements (multi-beam laser) capable of emitting a large number of laser light beams, a bias current Ibias flowing at the time of switching off has been heretofore set to be common to the plurality of semiconductor lasers in order to attain reduction in cost (e.g., see JP-A-09-272223). In the current drive type multi-beam laser driving device, variations in characteristic in the plurality of semiconductor lasers are considered so that the maximum current value selected from variations in current is set as the bias current Ibias common to all the semiconductor lasers when the modulating speed is given preference, and the minimum current value selected from variations in current is set as the bias current Ibias when prevention of abnormal lighting is given preference.
Incidentally, the drive current I of a semiconductor laser is generally given by the expression:I=Is * [exp{q(V−IR)/kT}−1]
in which Is is a backward saturation current, q is an elementary electric charge (charge of an electron), V is a drive voltage, R is the internal resistance of the semiconductor laser, k is a Boltzmann constant, and T is an absolute temperature.
In a low light intensity region in which the voltage drop by the internal resistance R is low, it is obvious from the characteristic graph shown in FIG. 11 that the drive current I changes exponentially according to the drive voltage V. Accordingly, in the voltage drive type driving device, because the drive current I, that is, the intensity of emitted light changes exponentially according to the change quantity ΔV of the drive voltage V if negative feedback control is performed to control the terminal voltage when the semiconductor laser is switched on, there is a problem that the gain of negative feedback control changes widely so as to make it difficult to perform stable control.
Even when the semiconductor laser is switched off, a voltage needs to be applied to the semiconductor laser in order to quickly switch off the semiconductor laser large in internal resistance R. The switching-off of the semiconductor laser can be achieved most simply if the applied voltage is made zero. In the edge emission laser, it is however necessary to supply a bias current Ibias near the emission threshold current to the semiconductor laser continuously for the purpose of high-speed modulation even at the time of switching off the semiconductor laser. Accordingly, a voltage corresponding to the current near the emission threshold current must be applied to the semiconductor laser when the semiconductor laser is switched off.
On the other hand, in the surface emission laser, it is unnecessary to supply the bias current Ibias for the purpose of high-speed modulation because the volume of a resonator is small. It is however preferable that a bias voltage enough to avoid laser oscillation is applied to the semiconductor laser when the semiconductor laser is switched off because the modulating speed can be increased from the point of view of circuitry as the amplitude of the drive voltage decreases. In the bias voltage applied at the time of switching off the semiconductor laser like at the time of switching on the semiconductor laser, it is however difficult to set the drive voltage because it is obvious from FIG. 11 that the drive current changes widely as the drive voltage changes slightly.
Even if the drive voltage can be set, it is difficult to keep the bias current Ibias proper on the basis of only the drive voltage without consideration of the current because the current varies according to temperature change when the bias voltage is set fixedly. As described above, it is preferable that, in order to quickly drive the semiconductor laser large in internal resistance, a drive voltage not higher than the threshold and near the ON voltage is applied to the semiconductor laser when the semiconductor laser is switched off. Controllability like that at the time of switching on the semiconductor laser however becomes an issue if the voltage needs to be directly controlled.
Variation in the intensity of emitted light according to temperature change is a more significant issue in the voltage drive system. The intensity of light emitted from the semiconductor laser is basically proportional to the drive current. The emission threshold current makes a large contribution to temperature compensation. Accordingly, when the semiconductor laser is driven by a current sufficiently larger than the emission threshold current, variation in the intensity of emitted light according to the temperature change is small enough to be negligible.
In the case of the voltage drive system, because the terminal voltage of the semiconductor laser has a negative temperature coefficient for the temperature when the current is kept constant, it is necessary to reduce the drive voltage according to the temperature change so that the intensity of light does not vary in spite of the temperature rise in the condition that the semiconductor laser is switched on. Conversely, in the condition that the semiconductor laser is switched off, the voltage at the time of switching on the semiconductor laser must be increased to be higher than the voltage at the time of switching off the semiconductor laser in accordance with the temperature fall so that the semiconductor laser can be switched on again with the same intensity of light because the temperature decreases at the time of switching off the semiconductor laser.
As described above, when the intensity of light is to be controlled by the voltage drive system with accuracy equal to that obtained by the current drive system, the drive voltage for the semiconductor laser must be changed according to the temperature of the semiconductor laser.
In the related-art current drive system suitable for driving a plurality of semiconductor lasers (multi-beam laser), when, for example, a bias current Ibias to flow in common to all the semiconductor lasers at the time of switching off the semiconductor lasers is set according to the semiconductor laser having the largest emission threshold current Ith, the bias current Ibias exceeds the emission threshold current Ith so that some semiconductor laser may be switched on continuously if variation in characteristic of the semiconductor lasers is large. It is therefore necessary to suppress variation in characteristic of the semiconductor lasers strictly. However, if the specification for the semiconductor laser is stringent, the situation that the semiconductor laser cannot be provided may inevitably occur because of increase in cost of the semiconductor laser and reduction in yield of the semiconductor lasers according to circumstances.
On the other hand, in the related-art voltage drive system suitable for driving GaN blue edge emission lasers or single mode surface emission lasers, it is necessary to provide voltage sources according to elements because the optimal values of the bias voltages applied at the time of switching off the semiconductor lasers are different according to the elements. In this case, it is necessary to provide capacitors of high capacitance according to the elements in order to produce voltage sources with impedance kept low up to a high frequency. Particularly when the driving device is assumed to be formed as an IC, it is necessary to provide such capacitors of high capacitance in the outside of the IC because provision of the capacitors in the inside of the IC causes increase in cost. It is however a matter of course that increase in cost is brought about because the capacitors provided in the outside of the IC are required.
Therefore, when one common voltage source is to be used in the same manner as one common current value set in the current drive system, the bias voltage must be controlled according to the maximum or minimum value of the thread current. When a common voltage value is to be set according to the maximum or minimum value in the same manner as in the current drive system, it is necessary to suppress variation in characteristic of the semiconductor lasers strictly. That is, when a multi-beam laser driving device is to be formed, a drive circuit for one semiconductor laser needs to be simplified as much as possible in order to attain reduction in cost. As is obvious from the above description, the performance of the semiconductor lasers is however sacrificed when the related-art voltage drive system is used.